1. Field of the Invention
The present invention generally relates to liquid crystal displays and more particularly to the arrangement of transfers or electrical contacts for supplying counter electrode potential to opposing transparent substrates of a liquid crystal display.
2. Description of Related Art
A liquid crystal display has two transparent substrates. Sealed between the transparent substrates are transparent electrodes, oriented films, nonlinear elements and liquid crystal material. Voltage is applied across the electrodes on the two transparent substrates to create an image.
In recent years, the mainstream in large liquid crystal displays has been the active matrix type, which uses a nonlinear element for each pixel. Large scale integrated circuits are used for driving signal lines in the active matrix type liquid crystal display (AM-LCD) and now have an output voltage that has been reduced to about 5 volts in order to increase the driving frequency and to lower the cost of the LSI chips. To use these 5 V LSIs, the AM-LCD driving method has also been changed from one which basically locks counter electrode potential (Vcom) to a certain reference potential to another which modulates counter electrode potential in response to the timing of output signals.
For the method of modulating counter electrode potential (Vcom), it is necessary to transfer the modulation signal for the transparent counter electrode with as short a time delay as possible. It is difficult to reduce the electrical specific resistance of the transparent counter electrode, formed of transparent conductor film such as ITO, in its forming process. Therefore, an increasing number of AM-LCDs have been employing a multi-transfer structure with an increased number of transfers, which has conventionally been provided only on the four corners of the LCD substrate, for electrically connecting the counter electrode substrate to the active matrix substrate, thus reducing effective time delay. This multiple transfer structure is particularly useful in large high definition liquid crystal displays.
FIG. 1 is a top view of an active matrix type liquid crystal display (AM-LCD) employing a conventional multi-transfer structure. In many applications, signal lines 17 are drawn alternately upward and downward to connect to the TABs (not shown) on which a signal line driving LSI is mounted, via signal line lead electrodes 18, in order to provide a vertically striped RGB arrangement. Scan lines 15 have a larger wiring pitch than that of signal lines 17, so they are led towards a single side (leftward in FIG. 1) to connect to the TABs (not shown) on which a scan line driving LSI is mounted, via scan line lead electrodes 16. On the other side (right), an opening is provided through seal 24 that serves as a liquid crystal inlet 14. Active matrix substrate 11 and counter electrode substrate 12 have been scribed with their end positions aligned.
The structure with the end positions of two transparent substrates aligned on the side of liquid crystal inlet 14 produces the significant advantages of reducing the panel size and eliminating extra materials. In addition, the structure ensures many other advantages, such as higher yield in glass scribing in the liquid crystal cell assembly process, reduced cost due to decreased amount of liquid crystal used in the liquid crystal feeding process, and improvements in yield and ability in the sealing process for the liquid crystal inlet using UV-hardened resin after liquid crystal feeding. In this structure, multiple transfers are located in each boundary of the TAB bundled to meet the number of outputs for both scan line and signal line sides, and in many applications, are provided counter electrode potential (Vcom) from peripheral circuit substrates via TAB through lines. On the other hand, the liquid crystal inlet side, which is scribed with the end positions of the two transparent substrates aligned., must have the portion of the transparent substrates which extends beyond the liquid crystal seal be small enough to obtain the above benefits. This limitation in space prevents transfers from being used on the side having the liquid crystal inlet. Thus, it has not been possible to apply counter electrode potential at the liquid crystal inlet side. This in turn has made it difficult to maintain uniform counter electrode potential across the display surface, resulting in detrimental effect upon display quality. This degradation in display quality is worse in liquid crystal displays with a large display area.
To form a number of transfers on the liquid crystal inlet side, in order to supply counter electrode potential (Vcom) at the liquid crystal inlet side, it is possible to extend the transparent substrate beyond the liquid crystal seal to provide sufficient space to form an electrode pattern similar to the pattern on the other sides, and connect wiring from external peripheral circuit substrates to this pattern. However, this would preclude the many advantages described above.